(628a) Regeneration of Ammonia-Loaded Metal Halide Absorbents

With more than 150 million tons production per year, ammonia is one of the most commonly used chemical in the world. The conventional Haber-Bosch process for ammonia synthesis requires high pressure to ensure suitable conversion.¹ Such high pressure makes the ammonia synthesis costly, energy- and capital-intensive.² We are developing an innovative process to synthesize green ammonia at lower pressure, with hydrogen from water, nitrogen from air, and wind-generated electricity.³ We seek to develop a process for lower pressure ammonia synthesis, because low pressure process is safer, cheaper, more robust, and more amenable to downscaling for distributed production from renewables.

In our proposed reaction-absorption process, we are synthesizing ammonia using a conventional catalyst and a column of solid absorbents.⁴ These absorbent materials – alkaline earth metal halides - have attractively high equilibrium absorption for ammonia by forming an ammine complex. These complexes have various coordination numbers, some of which stable even at temperatures as high as 400 °C (close to reaction temperature).^5,6 Metal halides can be reversibly loaded and unloaded with ammonia. We have already proved and reported the viability of high temperature ammonia absorption into metal halides, enabling low pressure ammonia synthesis.

In this talk, we are reporting some of our most recent findings regarding our search for a practical regeneration protocol to unload ammonia from the absorbent materials. Pressure and temperature swing desorptions were studied to figure out the best operating conditions. Results indicate that depending on the coordination number of the ammine complex, which is formed in the absorber, a combination of pressure and temperature swing is required. Vacuum desorption showed to be fast and reliable. Currently, we believe a full regeneration is not necessary. Thus, cyclical partial desorptions at high ammonia loadings is probably the solution for sustainable regeneration of absorbent materials.

References

(1) Vojvodic, A.; Medford, A. J.; Studt, F.; Abild-Pedersen, F.; Khan, T. S.; Bligaard, T.; Nørskov, J. K. Exploring the Limits: A Low-Pressure, Low-Temperature Haber–Bosch Process. Chem. Phys. Lett. 2014, 598, 108.

(2) Jennings, J. R. Catalytic Ammonia Synthesis, 1st ed.; Plenum Press, 1991.

(3) Reese, M.; Marquart, C.; Malmali, M.; Wagner, K.; Buchanan, E.; McCormick, A.; Cussler, E. L. Performance of a Small-Scale Haber Process. Ind. Eng. Chem. Res. 2016, 55 (13), 3742.

(4) Malmali, M.; Wei, Y.; McCormick, A.; Cussler, E. L. Ammonia Synthesis at Reduced Pressure via Reactive Separation. Ind. Eng. Chem. Res. 2016, 55 (33), 8922.

(5) Sørensen, R. Z.; Hummelshøj, J. S.; Klerke, A.; Reves, J. B.; Vegge, T.; Nørskov, J. K.; Christensen, C. H. Indirect, Reversible High-Density Hydrogen Storage in Compact Metal Ammine Salts. J. Am. Chem. Soc. 2008, 130 (27), 8660.

(6) Wagner, K.; Malmali, M.; Smith, C.; McCormick, A.; Cussler, E. L.; Zhu, M.; Seaton, N. C. A. Column Absorption for Reproducible Cyclic Separation in Small Scale Ammonia Synthesis. AIChE J. 2017.